thumbnailer: improve behavior for files with black frames on pos=0, with a length less than a minute and weirdos where we can't calculate position and length correctly. black screens should no longer be returned
thumbnailer: improve behavior for files with black frames on pos=0, with a length less than a minute and weirdos where we can't calculate position and length correctly. black screens should no longer be returned

kamcord/VLCKit,simon1984/test,larrytin/VLCKit,kamcord/VLCKit,kamcord/VLCKit,programming086/VLCKit,siuying/VLCKit,simon1984/test,waterskier2007/VLCKit,simon1984/test,waterskier2007/VLCKit,brion/VLCKit,hpsoar/VLCKit,siuying/VLCKit,CollabraMusic/VLCKit,strukturag/VLCKit,larrytin/VLCKit,i386/vlckit,CollabraMusic/VLCKit,hpsoar/VLCKit,strukturag/VLCKit,CollabraMusic/VLCKit,siuying/VLCKit,waterskier2007/VLCKit,programming086/VLCKit